1. Field of the Invention
The present invention relates generally to a method and apparatus for determining material properties using scanning microscopy. More specifically, a new method is taught for improving spatial resolution and accuracy of dopant density profiling of materials used in semiconductors when conducting scanning probe microscopy.
2. State of the Art
State of the art integrated circuit technology demonstrates that it is possible to create active and passive electrical and electronic components on a semiconductive substrate at the sub-micron level. This ability requires accurate knowledge of the spatial extent of impurity dopants that are incorporated into the semiconductive substrate. This knowledge is necessary because of the scale at which the concentration, and thus variation or profile of the dopants is operating. Essentially, in order to achieve predictability in active and passive component behavior, it is necessary to be able to accurately measure the dopant density profiles which can then be used by design engineers in design and manufacturing processes. Lack of precision in the incorporation of dopants can result in a proliferation of undesirable defects during later steps in a manufacturing process, and possibly less than adequate performance of the finished product.
Dopant regions are formed by actively injecting or passively diffusing a desired impurity into a surface of the substrate. When dealing with silicon as the semiconducting substrate, a native oxide occurs when the substrate surface is exposed to oxygen. It is possible to implant the dopants through this thin and naturally occurring oxide layer.
After forming active and/or passive components on the semiconductive substrate surface, functionality of the integrated circuit is determined by many factors. One important factor is the concentration of dopant atoms within dopant regions. Therefore, accurate profiling of the substrate is critical for accurate estimates of operating characteristics.
The state of the art is replete with different ways to characterize and thus create a profile of dopant regions. There are many one dimensional dopant profiling techniques, such as secondary ion mass spectroscopy (SIMS), spreading resistance, junction staining and anodic sectioning. Disadvantageously, these methods all fail to provide profiling in two dimensions. However, with the advent of the scanning tunneling microscope and the scanning probe microscope, new methods for dopant profiling became possible on a nanometer scale.
For example, early measurement techniques generally measured resistance and converted each resistance reading to a concentration amount.
Another technique is to profile a cross-section of the substrate which has been severed along the dopant region. Dopant concentration is then measured in two dimensions in both vertical and lateral directions.
Another way to obtain two-dimensional dopant profile measurement and inverse modeling is by scanning capacitance microscopy as disclosed in U.S. Pat. No. 5,523,700. This patent teaches how a one dimensional model is used to extract two dimensional dopant density profiles from measurements made by a scanning capacitance microscope.
Finally, one illustrated method as taught in the prior art is shown in FIG. 1. A probe 10 is placed on a surface 12 of a substrate material 14 to be probed. In this example, the substrate material being probed is silicon, with a layer of silicon dioxide 16 (naturally occurring or intentionally disposed thereon) covering the surface. The probe 10 is placed on and moved over the layer of silicon dioxide 16. In this cross-sectional view, the dopant density or carrier concentration is represented in the silicon 14 as concentration contours 18. In this method, the probe 10 is operated in what is referred to as a constant change in capacitance mode. A harmonic AC bias voltage is applied to the probe 10. By measuring the AC voltage necessary to maintain a constant change in depletion capacitance, it is possible to determine dopant concentration using a conversion model and algorithm. The conversion algorithm relates AC bias voltage data to dopant density concentration using a physical model.
The physical model requires particular parameters to be defined in order to accurately represent the system of the substrate 14. These parameters include an oxide dielectric constant, oxide thickness, probe tip radius, pining dopant density, pining bias voltage, and sensor probing voltage. Of these parameters, a free parameter can be the oxide dielectric constant, the oxide thickness, or the sensor probing voltage. The free parameter controls the lowest dopant density generated in the conversion. By fixing the dielectric constant and oxide thickness, it is possible to vary the sensor probing voltage for the best fit of SIMS.
FIG. 2 shows that disadvantageously, this method and model fails to take into account any dopant gradient which may exist in the material being probed. For example, the model (referred to as a first order model) incorrectly assumes that the dopant density is constant relative to the position of the probe in the material being probed. This is demonstrated by the depletion region 22 and the annular rings 24. This assumption leads to an AC bias voltage to dopant density physical model which suffers from this inaccuracy.
It is observed in FIG. 2 that for each annular ring, it is necessary to find the following: 1) the probe tip to silicon capacitance, 2) the oxide capacitance, 3) the silicon capacitance, and 4) the capacitance per annular ring 24 which is a series combination of the capacitances of steps 1, 2 and 3. The total capacitance is the sum contribution of all the annular rings 24.
It would be an advantage over the state of the art to provide a new method for determining dopant density profiling which improved spatial resolution and accuracy. It would be a further advantage to provide a method for improved dopant profiling which could account for gradients in dopant density within the substrate being scanned.